What phrase best completes the sentence: She was so mad at her boyfriend, she ____________ on him when he called.

The phrase "hung up" is the best choice to complete the sentence because it effectively conveys the action someone might take when they are angry during a phone call. It suggests a clear and decisive reaction to the emotional state of being mad, indicating that she abruptly ended the conversation as a result of her anger. This aligns with common behaviors when individuals feel frustration or hostility in a relationship context.

The other options do not fit as well within the context of the sentence. "Yelled" and "screamed" imply vocal expression of anger but do not indicate a termination of the conversation, which is vital to the context created by the phrase "mad at her boyfriend." "Spoke" is not appropriate in this context either, as it suggests a normal conversation rather than an action stemming from anger. Therefore, "hung up" is the most effective in reflecting the emotions described.